The present invention relates generally to an optical information recording and reproducing apparatus, and more particularly to a track seek method and device which are used for successively recognizing every current track position onto which the light beam is irradiated, and for positioning the light beam on a desired target track.
Optical disks and optical cards have conventionally been known as optical information recording media on which high-density recording and reproduction of information are performed by use of light. On the optical information recording media are provided a plurality of optically detectable guide tracks to function as a guide for recording or reproduction of desired information, and data tracks are each provided between every two adjacent guide tracks. Information is recorded in the form of "pits" each having a size of about 1 to 3 .mu.m by irradiation of a minutely converged light beam on the data track. For each of the data tracks is defined a unique track number.
To successively perform information recording or reproduction on such an optical information recording medium by moving the light beam from a current track number (i.e., a track onto which the light beam is currently being irradiated) to a new track number, the light beam must be moved in a direction across the guide tracks and positioned at the new or target track number. Each time the light beam crosses the guide track, reflection of the light beam from the guide track is received to be converted into an electrical signal. This electrical signal is amplified and shaped so as to generate a guide-track-crossing signal representative of the guide track crossing by the light beam. By counting the generated guide-track-crossing signals one by one, it is possible to successively recognize a specific track on which the light beam is being currently located. This can be used as a feedback value to move the light beam to the vicinity of the target track. A series of these operations is collectively called a "track seek" operation or technique. May examples of the prior art track seek technique are known from, for example, Japanese Patent Laid-open Publication Nos. SHO 56-134364 and SHO 62-154273.
If some flaw, scratch or dirt exists in or on the optical information recording medium, however, a light reception or guide-track-crossing signal of appropriate level can not be obtained when the light beam crosses the guide track. This would cause errors in the guide-track-crossing signals, so that the light beam tends to be wrongly positioned a considerable distance away from the target track. A prior art for addressing this problem is disclosed in Japanese Patent Publication NO. HEI 4-50676, for example.
The disclosed prior technique will be outlined with reference to a timing chart of FIG. 4. In item (a) of FIG. 4, solid line indicates actually detected guide-track-crossing signals, while dotted line indicates a missing guide-track-crossing signal that has not been actually detected although it should have been detected. Item (b) of FIG. 4 indicates an imitation signal to be added, and item (c) of FIG. 4 indicates corrected guide-track-crossing signals that are obtained by addition of the signals of items (a) and (b). In the case where no guide-track-crossing signal is not detected within a preset time period after detection of a last guide-track-crossing signal as the light beam is moved at a constant speed or at a given acceleration in a direction across the guide tracks, it is considered that some flaw, scratch or dirt exists in or on the optical information recording medium, and then an imitation signal as indicated in item (b) is generated in place of the missing guide-track-crossing signal. Thus, by counting together the guide-track-crossing signals and the imitation signal as new guide-track-crossing signals as shown in item (c), it is allowed to move the light beam to the vicinity of a desired target track. The above-mentioned preset time period is a period allowing for time differences between adjacent guide-track-crossing signals and is represented as "T3" that equals T1+(T1-T2). This time period is used to deal with scanning speed variations of the light beam to some degree.
However, large-size flaw, scratch or dirt in or on the optical information recording medium may extend over two or more guide tracks, and the above-noted prior technique was often unable to properly deal with the presence of such large-size flaw, scratch or dirt particularly when the light beam was accelerating or decelerating over such places. That is, since the prior technique is based on generating of the imitation signal when the track-crossing signal has failed to be detected within the preset time period, modification of the preset time period in consideration of the last period in the above-mentioned manner is permitted only when only one guide track-crossing signal is missing. Therefore, when plural guide-track-crossing signals have failed to be detected in succession, there is no other way than to successively generate imitation signals while repeating the same time period. But, in such a case where the light beam is accelerating or decelerating, the same preset time period gets out of correspondence to the actual track crossing, and there would be caused an error in the number of imitation signals. Consequently, the number of the guide-track-crossing signals as modified by addition of the imitation signals would greatly differ from the actual number of the guide-track-crossing signals, and therefore the light beam could not be accurately moved to be positioned in the vicinity of a desired target track. In particular, if large-size flaw, scratch or dirt is present in or on the optical information recording medium at time points before and after the light beam is shifted from the acceleration state to the deceleration state, there will be caused an error in the number of added imitation signals, which will present significant problems in the seek track operation.
Further, in order to reduce the time required for the control seek operation, it is desirable that the movement/scanning of the light beam should be changed to an optimum speed as necessary rather than being at a constant seed or constant acceleration. But, the above-mentioned prior technique lacks reliability due to the previously-stated reasons and hence is not suitable for varying the light beam scanning speed as desired during the control seek operation.